Dean, could you provide your print-related preferences? This is what you should do:
1) Visit the URL "about:config". This should display all of your current preferences, along with a text box labelled "Filter:".
2) In the filter text box, enter "print." including the period. Firefox should limit the display to prefs containing "print."
3) Attach the resulting list to this bug report.

Verified that the /etc/cups/brother.ppd file contains page size options with entries like BrA4/A4. If the file is edited so that the 'Br' strings are removed, then the driver works ok. It seems that firefox, just needs to filterthe file accordingly...

(In reply to comment #5)
> do you see this issue still with version 3.6 or 4.0 beta?
No, I haven't seen this problem for four years or so. (It was a really a Brother ppd issue rather than a firefox issue).